In the prior art, various heat transfer methods have been known, and among them, there has been practiced the sublimation transfer method in which a sublimatable dye is used as the recording agent and is carried on a substrate sheet such as paper to provide a heat transfer sheet, which is superposed on a transferable material dyeable with a sublimable dye such as a fabric made of polyester. Heat energy is then imparted in a pattern from the back surface of the heat transfer sheet to cause migration of the sublimatable dye to the transferable material.
In the above sublimation transfer method, in the sublimation printing method wherein the heat transferable material is, for example, a fabric made of polyester, etc., heat energy is imparted for a relatively longer time, whereby the transferable material itself is heated by the imparted heat energy. Consequently relatively good migration of the dye can be obtained.
However, with progress in recording methods, in the case of using a thermal head, etc., and forming fine letters, figures or photographic images on, for example, transferable materials having dye receiving layers formed on polyester sheets or papers, at high speed, heat energy must be imparted within a matter of seconds or less. Therefore, since the sublimatable dye and the transferable material cannot be sufficiently heated, images with sufficient density cannot be formed.
Accordingly, in compliance with such high speed recording requirements, sublimable dyes of excellent sublimation were developed. However, dyes of excellent sublimation generally have small molecular weights, and therefore the dyes may migrate with lapse of time in the transferable material after transfer, or they may bleed out on the surface, whereby there ensue problems such as an elaborately formed image being disturbed, becoming indistinct or contaminating surrounding articles.
If sublimable dyes with relatively larger molecular weights are used in order to circumvent such problems, the sublimation speed is inferior in the high speed recording method as mentioned above. Therefore, images with satisfactory density could not be formed as described above.
Accordingly, in the heat transfer method by the use of a sublimable dye, it has been strongly desired under the present situation to develop a heat transfer sheet which can give sharp images with sufficient density and yet exhibit excellent adhesiveness of the image formed by imparting heat energy within a very short period of time as mentioned above.